In the field of measuring, detecting and/or analyzing the characteristics of gaseous or liquid mixtures, there's a heavy reliance placed on the evaluation of the absorption spectra that are obtained by use of optical methods. Because of the increased awareness on the part of society as a whole and further because of increased government regulatory activities, this field of monitoring and/or analyzing the various gaseous mixtures that are present in industrial, residential, or commercial environments has been subjected to greater scrutiny and has been the source of increased interest so that more accurate and efficient methods can be developed to detect and subsequently reduce the effects of such gases that may be harmful to persons as well as to the environment. Some of the various gaseous mixtures that are of concern come about as by-products of processes or operations that are essential to society, as for instance, the use of automobiles or the burning of fossil fuels to generate electricity where the concerns are the efficient burning of those hydrocarbon fuels. Consequently, it is obvious that the means for dealing with such gaseous mixtures is not the elimination of the processes that generate them, but instead, in detecting and monitoring these gaseous mixtures and taking steps to contain them so that their effects can be minimized, or in fact neutralized altogether. Examples of some of the gaseous mixtures that are recognized as harmful or where the absence of which may be harmful, are; sulfur dioxide (SO.sub.2), ozone (O.sub.3), carbon dioxide (CO.sub.2), nitric oxide (NO), nitrogen dioxide (NO.sub.2), and ammonia (NH.sub.3).
An example of an industrial setting where it is necessary to detect these as well as other gaseous mixtures is in the field of monitoring and controlling stack gas pollutants in a combustion control environment. Solid electrolyte compositions which are uniquely responsive to certain gaseous mixtures have been utilized for this type of application, an example of which can be found in U.S. Pat. No. 3,915,830 which issued to A. O. Isenberg on Oct. 28, 1975. In some measurement systems of this type, a sensing electrode which is contacted by the stack gas emissions to be monitored is disposed on one side of the solid electrolyte cell while a reference electrode, which is contacted by reference gas, is disposed on the opposite side of the solid electrolyte cell. An EMF signal is generated which responds to the difference in partial pressure in the gas specie across the electrolyte. This type of approach may require continuous operator monitoring because of the fact that the reference gas may not always be adequately isolated from the stack gas so as to insure that the integrity of the reference gas is maintained to the precise degree necessary. Since the measurement of the stack gas is made relative to the reference gas, it is therefore essential to constantly monitor the reference gas and recalibrate when necessary.
Another approach to the detection and/or measurement of gaseous mixtures is the use of absorption spectroscopic techniques which utilize the fact that, at specific wavelengths of electromagnetic radiation, certain gases exhibit specific absorption characteristics which can be used to identify and quantify particular constituents of that gaseous mixture.
An example of the use of spectrographic techniques for the detection and/or measurement of gaseous mixtures involves the use of a device known as an acousto-optic tunable filter, commonly known as an AOTF. U.S. Pat. No. 3,805,196 which issued to J. D. Feichtner et al. on Apr. 16, 1974 discloses the use of a crystal made of thallium arsenic selenide (TAS) which can be operated in the infrared region of the electromagnetic spectrum to act as an AOTF. Depending on the geometry of the crystal and the RF signal that is used for modulation, the AOTF can be effectively operated in conjunction with the detector which detects the absorption characteristics of the gaseous mixture through which an infrared beam is directed, to achieve the detection of the various gases which are of concern. An example of an application of AOTF technology can be found in U.S. Pat. No. 4,505,550 which issued to K. B. Steinbruegge on Mar. 19, 1985. In this patent, input and output polarizers are coupled to and aligned with the AOTF device so as to attain the precise absorption band center for the gaseous constituents. Another example of an AOTF technology in the field of gaseous mixture detection and monitoring can be found in U.S. Pat. No. 4,652,756 which issued to F. M. Ryan et al. on Mar. 24, 1987. In this patent, the tuning function of an AOTF device is utilized in combination with a source of radiation that produces pulsed light at predetermined wavelengths. A detector, placed across the environment of interest which in this example can be a gas stack, can discriminate between the pulsed light emissions and the steady thermal emissions from the hot gas stack.
Though the use of AOTF devices for the purposes of detecting and/or analyzing gases of interest has been effective in a large number of industrial and commercial applications, the sensitivity of this technology has not reached the level that is now becoming desirable in order to meet requirements of environmental regulations which have been becoming more strict. For instance, if a detection arrangement could be developed that could measure an amount of SO.sub.2 in an environment of interest at a level of 10 ppB, such a detection arrangement could easily meet present and proposed environmental regulations. Although there are presently certain types of gas measurement arrangements in existence, which are capable of operating in this range, such arrangements suffer from deficiencies such as an inability to be modified so as to operate for a different gaseous mixture, or where such an arrangement can differentiate gases, it is subject to interference because of the filtering arrangement being used. Specifically it is possible to detect SO.sub.2 at this level with an arrangement utilizing an ultraviolet induced fluorescence technique, however, such an arrangement is limited to the application whereby it is desired to detect and quantify SO.sub.2 only, it is not effective for other gases that may be of interest. Additionally, it is possible to use an ultraviolet absorption technique which can be tailored to suit other gases of interest but such technique uses a filter wheel to achieve the specific bandwidth associated with the particular gas of interest. In this approach, because of the limitation of using a filter wheel, it is necessary to operate in a low rotating frequency range which has the disadvantage of increasing the effect of detector noise. The filter wheel approach has the further disadvantage that it measures the relative amounts of light absorbed in two neighboring wavelength ranges and deduces therefrom, the concentration of the absorbing gas present. This approach is not specific to the desired gas however, so that any other absorbing species at these wavelengths will produce an interfering absorption and hence, an erroneous measurement.
Still another technique utilizes an interferometer, or as it is sometimes referred to in the industry, an etalon, to measure the gaseous mixture constituents through the selective transmission of the periodic spectra associated with the gaseous mixture of interest. An example of such a technique can be found in U.S. Pat. No. 3,939,348 which issued to J. J. Barrett on Feb. 17, 1976. In this patent, a Fabry-Perot interferometer is used to provide a plurality of transmission windows regularly spaced in frequency. Selectively separated periodic spectra which are made up of a plurality of rotational, vibrational infrared absorption lines associated with the gaseous mixture of interest, are transmitted in the form of fringes thereby providing a detectable signal from which a determination of the amount of the particular gas of interest can be made. The Fabry-Perot interferometer which is essential to the operation of this arrangement, provides for a mirror separation which can be adjusted to simultaneously transmit all of the rotational, vibrational infrared absorption lines of a molecular species of the gas of interest. This approach to gaseous mixture measurement and detection has provided an advantage in that the sensitivity achieved has been an advance over existing techniques, however, by relying on a mechanical arrangement for providing the selective separation of the periodic spectra, this approach suffers from certain limitations inherent in the use of a mechanical filtering arrangement. For instance, the accuracy and hence, the sensitivity of this approach is dependent on the ability to accurately align the mirror elements of the Fabry-Perot interferometer to the precise bandwidth desired. Additionally inherent in the operation of such a mechanical arrangement is the limitation that modifying the operating characteristics of this measurement technique requires a cumbersome and time consuming manual operation involving the actual alignment or tuning of the mirror separation and the verification of the results of this alignment.
Another example of the use of an interferometer device for the detection and/or measurement of a particular gas of interest can be found in British Patent No. 2,174,198 issued to G. Fortunato on Oct. 29, 1986. In this patent, rather than using a tunable Fabry-Perot interferometer, a stress tunable birefringent etalon is used to achieve the selective separation of the periodic rotational vibrational infrared absorption spectra associated with the particular gas of interest. The modulation arrangement of this approach achieves the specific bandwidth by use of a photoelastic element which is excited by a piezoelectric ceramic so that the birefringence is variable by compression. French Patents 2,555,747 and 2,555,748 issued on May 31, 1985 to Fortunato et al. also employ interferometric techniques; in Patent No. 2,555,747, a piezoelectric element is used to modulate a luminous beam and to provide temperature compensation and in Patent No. 2,555,748 a rotating polarizer is used as a modulation technique.